Effect of C-Terminal Amidation on Folding and Disulfide-Pairing of α-Conotoxin ImI

Solution structure of a novel α-conotoxin with a distinctive loop spacing pattern

α-Pharmacological conotoxins are among the most selective ligands of nicotinic acetylcholine receptors with typical cysteine frameworks. They are characterized by the intercysteine loop and classified into various subfamilies, such as α3/5 and α4/7 conotoxins. A novel α-conotoxin, Pu14a (DCPPHPVPGMHKCVCLKTC), with a distinct loop spacing pattern between cysteines was reported recently. Pu14a belongs to the Cys framework 14 (-C-C-C-C) family containing four proline residues in the loop 1 region. Similar to another framework 14 conotoxin Lt14a (MCPPLCKPSCTNC-NH2), Pu14a has C1-C3/C2-C4 disulfide linkage, and can inhibit some subtypes of nicotinic acetylcholine receptors. In this study, the solution structure of Pu14a was investigated using 1H nuclear magnetic resonance spectroscopy to understand the structure-activity relationship of this conotoxin. 20 converged structures of this conopeptide, with RMSD value of 0.77 Å, were obtained based on distance constraints, dihedral angles and disulfide bond constraints. The three-dimensional structure of Pu14a showed remarkable difference from typical α-conotoxins because of a large intercysteine loop between C2 and C13, as well as a 3(10)-helix near the C-terminal. Furthermore, four proline residues in Pu14a adopted the trans conformation that may correlate with the large loop configuration and the biological activity of this conopeptide. The distinct structural characteristics of Pu14a will be very useful for studying the structure-activity relationship of α-conotoxins.

Helix Triangle: Unique Peptide-Based Molecular Architecture

We here report a unique cyclic peptide structure, "helix triangle", as a unique example of peptide-based molecular architecture. The cyclic peptide is designed to have a triangular shape in which three 9mer helical peptide units make the sides and three pyrene derivatives make the apexes. The helical peptide units are ideally linear, and the pyrene units are ideal 60 degrees angular components. The yield of the cyclic peptide was relatively high despite its large cycle size. Absorption and fluorescence spectroscopy revealed that the three pyrene units do not interact with each other electronically, and circular dichroism spectroscopy indicated that the helical peptide units take 3(10)-helical conformation. Geometry optimization by the semi-empirical molecular orbital method gave a triangular structure with 3(10)-helices as the plausible molecular structure. To gain more information on the geometry and demonstrate one example of its self-assemblies, the monolayer of the cyclic peptide was prepared at the air/water interface, and its surface pressure-molecular area isotherm was studied. The isotherm indicated formation of a stable monolayer and suggested that the cyclic peptide actually takes the triangular structure predicted by the geometry optimization. The monolayer was then transferred onto a substrate and characterized by various methods. Ellipsometry and infrared reflection-absorption spectroscopy confirmed that the cyclic peptide has horizontal orientation to the surface in the monolayer. Furthermore, absorption and fluorescence spectroscopy showed that the isolated electronic properties of the pyrene units are intact even in a condensed state in the monolayer.